1. Field of the Invention
This invention relates to a mounting structure for a magnetic encoder, and in particular, to an improvement in a mounting structure for the magneto-sensitive element of a magnetic encoder.
Pertaining to such a mounting structure, this invention particularly relates to an improvement in its wiring structure.
2. Description of the Related Art
As a result of the recent technical improvement in the magnetic-reluctance-effect element as a magneto-sensitive element, it has become the common practice to employ a relatively expensive high-resolution type magnetic encoder in general equipment. The most typical employment of such a magnetic encoder is to attach it to the end surface of a rotary motor.
In the following, a typical conventional magnetic encoder of this type will be described with reference to FIGS. 8 to 10.
FIG. 8 is a sectional view showing the essential part of the encoder, which has a magnetic drum 1, constituting a magnetic scale, whose peripheral surface is continuously magnetized through horizontal magnetization at, for example, equal pitches. The magnetization pattern (not shown) is such that the adjacent magnetic-pole pairs are of opposed polarity as N,N,S,S,N,N, . . . and arranged parallel with respect to the direction of movement, i.e., the direction of rotation. A rotating shaft 3, which is a movable component, protrudes from a rotating machine 2 consisting of a motor or the like, which is the object of the monitoring. This rotating shaft 3 is fixed to the center of the magnetic drum 1. Formed on that end surface of the rotating machine 2 to which this encoder is attached, is a step section 2a, into which a ring-like base member 4 is screwed. The upper surface of this base member 4 constitutes a reference plane 4a, which is perpendicular to the axis of the rotating shaft 3. Screwed to this reference plane 4a is a retaining member 5. A magnetic sensor 6 having magnetic-reluctance-effect elements is fixed to this retaining member 5 by means of a fluid additive. The magneto-sensitive plane of this magnetic sensor 6 faces the peripheral surface of the magnetic drum 1, on which the magnetic pattern is provided, with a predetermined gap (a magnetic gap g which is, for example, 100 .mu.m) therebetween.
The encoder includes a printed board 7. Mounted on this printed board 7 is a circuit part 11, which processes electric signals emitted from the magnetic sensor 6. This printed board 7 is placed over the upper end surface of the magnetic drum 1 by means of a screw 12, which is inserted through a support cylinder 9 before it is screwed into the base member 4, so that the printed board 7 is not in contact with other parts. The printed board 7 is connected to the magnetic sensor 6 through a lead wire 8. An external wiring cable (not shown) extends from the printed board 7 for the electrical connection with an external circuit.
A cover member 10, which covers these encoder parts and through which the external wiring cable can extend to the external circuit, is attached to that end surface of the rotating machine 2 to which these parts are attached, in such a manner that no dust is allowed to enter.
The magnetic-reluctance-effect elements, which constitute the magnetic sensor 6, are grouped into two element groups. Supposing the magnetization pattern is at equal pitches p, these two element groups are shifted from each other by p(n+1/4) (n is an integral number). Each of these magnetic-reluctance-effect elements extends in a direction which is perpendicular to the magnetic path of the magnetic pattern, and exhibits form magnetic anisotropy when a bias current flows in the direction in which each of them extends. One rotation of the magnetic drum 1 causes the electrical resistance in each current path to change in correspondence with the density of the magnetic flux resulting from the magnetic pattern, which is perpendicular to the longitudinal direction in which each magnetic-reluctance-effect element extends. Then, the magnetic sensor 6 generates a pair of detection signals which retain an electrical-angle phase difference of 90.degree.. These detection signals are transmitted through the lead wire 8 to the printed board 7, where they are processed by processing circuits provided in the printed board 7, including an amplifier, a waveform shaping circuit and an impedance matching circuit, before they are transmitted to the external circuit as output signals, which are in the form of a pair of incremental square-wave pulses with an electrical-angle phase difference of 90.degree.. The external circuit, which receives these output signals, discerns the direction of rotation from the leading edge condition of these square waves, and the amount of rotation from the number of pulses.
The construction of a conventional magnetic encoder of the above-described type, which has the external wiring cable for electrical connection with the external circuit, will be described with reference to FIG. 9, which is a sectional view showing the essential part of the encoder, and FIG. 10, which is an enlarged sectional view of the same.
As shown in FIG. 9, the housing 4 is fixed to one end surface of the motor case 2c. The magnetic drum 1 is fixed to the end of the rotating shaft 3, which protrudes through an opening provided at the center of the housing 4. The retaining member 5 is screwed to the mounting surface of the housing 4, which is perpendicular to the axis of the rotating shaft 3. Fixed to this retaining member 5 is the magneto-sensitive element 6, which faces the magnetized peripheral surface of the magnetic drum 1, with a small gap therebetween.
The printed board 7, on which circuit parts 11 for signal processing are mounted, is firmly supported by the housing 4 in such a manner that it faces the upper end surface of the magnetic drum 1 without being in contact with other parts. The printed board 7 and the magneto-sensitive element 6 are connected to each other through the flexible flat cable 8. The printed board 7 is connected to the external circuit through a wire harness 13, which constitutes the external wiring member.
Situated over the motor end surface to which these encoder parts are attached, is the cover member 10, which is fixed to the peripheral surface of the motor case 2c. This cover member 10 has in its upper section a hole 10a for external wiring. Fitted into this hole 10a is a bush 14, through which the wire harness 13 is passed, the bush 14 being made of an elastic material and constituting a protecting member. This bush 14 is attached to the inner periphery of the hole 10a so as to protect the wire harness 13 from the edges of the hole 10a, which would otherwise damage the wire harness 13 when external stresses are applied to it, resulting in problems such as disconnection. This wire harness 13 is secured inside the cover member 10 by means of a bundle holder. This bundle holder, which is provided inside the cover member 10, abuts against the bush 14 so that no force is directly applied to the connecting section between the printed board 7 and the wire harness 13 when the wire harness 13 is pulled on the outside.
The cylindrical bush 14 has on its outer periphery a groove, which covers, when the bush 14 attached to the cover member 10, the entire inner periphery of the hole 10a, engaging with that portion of the cover member 10 which is around the hole 10a so that the bush 14 may not be easily detached from the hole 10a.
FIG. 10 shows the construction of another type of conventional external wiring structure, which is an improvement over the above-described one in terms of the ease with which the mounting and maintenance operations can be conducted.
As shown in FIG. 10, a cable 13a, which consists of a plurality of lead wires bundled together and outwardly covered, extends to the external circuit through a U-shaped cutout 10c provided at the motor side end of a cover member 10b, which covers the encoder parts attached to the end surface of the motor. A bush 14a, which is made of a flexible material and which is formed as an integral part, is provided on that outer peripheral portion of the cable 13a which is situated in the cutout 10c. Provided on the outer periphery of this bush 14a is a groove, which is to be engaged with the cutout 10c so as to prevent external tensile forces applied to the cable 13a from being directly applied to the connecting section between the printed board 7 and the cable 13a. The cover member 10b is fixed to the motor after the groove of the bush 14a, which is attached to the cable 13a at a predetermined position, is engaged with the cutout 10c. Thus, the bush 14a protects the cable 13a and the bush 14a is prevented from being detached.
Conventional encoders of the above-described types, however, have the following problems: the speediest way of conducting the operation of positioning the magnetic sensor 6 with respect to the retaining member 5 and fixing the former to the latter, is to adopt the method in which a jig is used. That is, the magnetic sensor 6 is held against the retaining member 5 at a predetermined position by means of a jig, with an ultraviolet-ray-curing-type adhesive applied therebetween. The two above components are fixed to each other by curing the adhesive through the application of ultraviolet rays. The jig has to continue to perform the above operation until the curing has been completed, which means a great number of jigs are necessary for mass production of such conventional encoders. In addition, this method involves the operation of removing the jig used when holding the magnetic sensor 6, with the result that a large space has to be utilized when conducting the mounting operation. Furthermore, the time needed for removing the jig after the curing has been completed has to be taken into account.
Accordingly, it is a first object of this invention to provide a magnetic encoder which can be manufactured without using a jig in order to temporarily retain the magnetic sensor.
Apart from the above problems, conventional wiring structures have the following problems: as described above, the bush 14, which is a separate component, is fitted into the hole 10a, and the wire harness 13 is passed through the bush 14, the wire harness 13 afterwards being secured by means of a bundle holder. Thus, this type of structure requires a large number of parts. Moreover, a lot of time has to be spent to engage the hole 10a with the groove of the bush 14, or shift the cover member 10 so that it may not be in the way when the wire harness 13 is connected to the printed board 7. In view of this, the structure of the above-described improvement, in which the groove of the bush 14a is slid into the U-shaped cutout 10c provided at the opening end of the cover member 10b, is advantageous since in this way the mounting time can be shortened. Furthermore, the structure which has a cable 13a as the external wiring member and in which the bush 14a is integrally formed around the cable 13a, is advantageous in that the cable can be secured more positively. Further, it obviates the operation of passing the cable through the bush, thus further shortening the mounting time. Moreover, it is advantageous in that the bush 14a can be mounted after the cable 13a has been connected to the printed board 7. This improvement over the conventional external wiring structures, however, is not without its problems. The operation of mounting the bush 14a is conducted on the cover member 10b. That is, the bush 14a, which is provided on the outer periphery of the cable 13a, has to be mounted on the cover member 10b while the groove of the bush 14a is slid along the U-shaped cutout 10c, which means this structure cannot be applied to larger motors. In addition, the structure is disadvantageous in terms of operational efficiency since it requires skill for to adjust operational power, for the operator has to hold the bush 14a, whose length is limited by the cable 13, with one hand, and the cover member 10b with the other hand.
Further, in the case where the bush 14a is integrally formed, the mounting operation area for the bush 14a is situated nearer to the motor side. If this condition is to be eliminated, it is necessary to lengthen that section of the cable which extends between the connecting section of the cable 13a and the position where the bush 14a is fixed. Accordingly, the magnetic encoder has to be a large one.
In view of this, it is a second object of this invention to provide a magnetic encoder which can be manufactured through a one-way mounting operation, which can be made smaller, and which has an external wiring structure that is of high quality and thus highly reliable.